The present invention relates to fluid controllers, and more particularly to fluid controllers wherein fluid pressure is used for moving a valve stem upward and downward.
Fluid controllers are already known wherein a piston attached to the upper end of a valve stem is driven by fluid pressure to move the valve stem upward and downward and thereby open and close a fluid channel. FIG. 3 shows an example of such a fluid controller, which comprises a valve body 1, a bonnet 2 secured to the upper end of the valve body 1, a cylinder 3 fixed to the upper end of the bonnet 2 and having an upward opening, a cylinder head 4 fitted over the cylinder 3, a valve stem 5 in the form of a solid cylinder and having a lower portion positioned in the valve body 1, an intermediate portion positioned inside the bonnet 2 and an upper portion positioned inside the cylinder 3, a valve element 6 provided at the lower end of the valve stem 5, a piston 7 mounted on the upper end of the valve stem 5, an annular flange 8 secured to the intermediate portion of the valve stem 5, and a coiled compression spring 9 bearing on the top wall of the bonnet 2 for biasing the valve stem 5 downward.
When the fluid controller is in the usual state, the valve stem 5 is held depressed by the elastic force of the compression spring 9, closing a communication passageway 12 between an inlet passageway 10 and an outlet passageway 11. The communication passageway 12 between the passageways 10 and 11 is opened by a fluid flowing into a cylinder chamber 14 through a working fluid inflow passageway 13 in the cylinder 3 and pushing up the piston 7.
The piston 7 is connected to the valve stem 5, for example, by an E-ring 16 fitted in an annular groove 15 formed in the valve stem 5 at the portion thereof projecting upward beyond the piston 7.
FIG. 4 shows on an enlarged scale the portion of the piston 7 connected to the valve stem 5. The E-ring 16 comprises a C-shaped member 17, and three claws 18 integral with the inner periphery of the member and each having an inner end fitted in the annular groove 15.
Alternatively, a U-ring 19 is used in place of the E-ring 16. FIG. 5 shows the U-ring 19. Base portions of bifurcated part of the ring 19 are fitted in the annular groove 15 formed in the valve stem 5.
The E-ring 16 and the U-ring 19 have a spring property, are fittable around the valve stem 5 when spread out at the opening, and are prevented from slipping off on contracting at the opening after fitting around the stem. Thus, the piston 7 is connected to the valve stem 5, and the fluid pressure acting on the piston 7 is transmitted to the valve stem 5 through the ring 16 or 19.
The conventional fluid controller has the problem that when the piston is subjected to a pressure greater than the prescribed pressure, a shearing force due to the fluid pressure acting on the piston breaks the E-ring or U-ring connecting the piston to the valve stem. Even if it is attempted to give an increased strength to the ring by using a different material for the ring, the ring material is not selectable as desired because of the limiting requirement that the ring needs to have a spring property, consequently imposing a limitation on the improvement of durability of the connection between the valve stem and the piston.